This invention relates to a method for making a CRT (cathode-ray tube) and particularly to a novel method of treating a partially-completed virgin CRT prior to sealing the mount assembly into the bulb.
As used herein, a virgin CRT is one which has never been exhausted, sealed, and gettered, and hence has no residual getter material present therein.
A shadow-mask color-television picture tube is a CRT which is usually prepared by steps including (a) producing a luminescent viewing screen structure upon the inner surface of a faceplate panel, (b) sealing the panel to the large opening of a funnel, (c) sealing a mount assembly into the small opening or neck of the funnel, and then (d) baking, exhausting, gettering and sealing the tube.
The electron emissions from the cathodes in the mount assemblies of some tubes were found to decrease rapidly with the amount of time the tubes were operated. It is known that the emissions from the cathodes in gassy tubes decrease in this way. Such decreases are attributed to destructive bombardment of the cathode by ions in the tube and/or by the deposition on the cathode of carbon from the decomposition of hydrocarbon gases in the tube.
In the art of making new or virgin cathode-ray tubes, various bulb treatments that are applied before sealing the mount assembly into the bulb have been suggested for various purposes. U.S. Pat. No. 3,589,791 to J. A. Kanellopoulos discloses injecting water into the bulb between the panel-sealing and mount-sealing steps in order to increase the humidity in the bulb, which increases the latitude of the subsequent CRT fabrication processes. U.S. Pat. No. 3,658,401 to J. A. Files discloses flushing the bulb after panel sealing with dry noncontaminating gas in order to reduce the rate of depletion of the getter during the operation of the finished CRT. U.S. Pat. No. 3,932,011 to J. J. Piascinski discloses flushing the bulb while still hot from panel sealing with a dry noncontaminating gas, reflushing the bulb after it has cooled with a dry noncontaminating gas and then temporarily capping the bulb to prevent degradation of the viewing screen when the bulb must wait a long time in potentially high humidities before it is exhausted and sealed.
In the art of rebuilding used cathode-ray tubes, the mount assembly is removed and replaced with a new mount assembly. This process is also called regunning. U.S. Pat. No. 3,063,777 to A. M. Trax discloses the step, when removing the mount assembly from the used CRT, of filling the bulb with a dry noncontaminating gas and then regunning the bulb in this atmosphere in order to avoid the baking normally done during the exhausting operation. U.S. Pat. No. 4,048,545 to T. H. Madden et al discloses, after removing the mount assembly from the used CRT, flushing the bulb with wet carbon-dioxide gas until the residual barium metal getter material is converted to barium carbonate, and not to other barium compounds which, by a complex series of chemical reactions, cause decreases in electron emissions from the cathodes of the rebuilt tube due to ion bombardment and/or deposition of carbon from the decomposition of hydrocarbon gases in the tube.